With the ever decreasing flying distance between the head and the disc in direct access storage devices, the dynamics of the head/disc interface need to be carefully characterized in order to mitigate failures due to, e.g., head crashes and dynamic "dings". It is imperative that the information be acquired in a real time manner. Currently, information has been acquired in one of four ways: 1) Measuring variations in capacitance between the head and the disc; 2) Performing "Wearscans" where permanent information has been imprinted on each track which is read at a later time to detect changes in signal amplitude; 3) Performing "Quantitative Readback Signal" (QRS) where a sinewave signal which was written on a spin-down of the disc is measured, as a function of disc velocity, on spin-up to determine the flying height of the head; and 4) Measuring variations in acceleration of the head with piezoelectric transducers. A variation of QRS mentioned in 3 above is to take the ratio of the third and first harmonic of the signal using a phase locked loop to determine the flying height of the head. This method is called "Harmonic Ratio Flying-height Analysis" (HRF).
Limitations of these method include: susceptibility to electrical noise, particularly in the case of the capacitance approach, non-linear response of piezoelectric transducers and of the capacitance measurement methods, inaccuracy in the capacitance measurement due to inherent variation (e.g., coating imperfections) which manifests themselves as variations in the dielectric properties of the disc surface or capacitance changes that are not caused by mechanical interface properties (e.g., surface pits), and incompatibility with real time measurement as in the case of the Wearscan, the QRS and the HRF techniques.
In U.S. Pat. No. 3,695,767 which issued on Oct. 3, 1972 to D. J. George et al., an apparatus for measuring the flying angle of a pneumatically supported magnetic head relative to a recording disc is described in which a laser beam is directed toward a transparent rotating disc. A portion of the laser beam is reflected from the surface of the disc at a first angle and a portion of the laser beam passing through the disc is reflected by the surface of the flying head which is reflected at a second angle. The two reflected beams will be at an angle relative to each other which is twice the angle between the two reflecting surfaces. A screen is disclosed positioned at a distance R from the flying head which intercepts the two reflected beams. Typically, the distance R is on the order of 50 feet so that the displacement distance on the screen for typical angles of .alpha. would be a few centimeters.
In U.S. Pat. No. 4,593,368 which issued on Jun. 3, 1986 to D. A. Fridge et al., a transparent disc is substituted for the normal hard magnetic disc in a storage system, and a broad spectrum of light such as from a Xenon lamp is directed substantially normal to the surface of the transparent disc, through the disc and onto the slider supporting the flying head. The light reflected from the slider and from the surface of the disc close to the slider is combined and spectrally analyzed for constructive and destructive interference at each of a large number of wavelengths to provide a plot of intensity versus wavelength.
In U.S. Pat. No. 4,201,475 which issued on May 6, 1980 to V. Bodlaj, a system is disclosed for non-contact distance or thickness measurement using a laser and a laser beam deflector for periodically deflecting the laser beam. The deflected laser beam scans an object and produces a measuring impulse during a forward and reverse motion of the scan when the laser beam hits a reference plane and a point on an object and is reflected along a line of sight to a measuring detector which is prepositioned with respect to the object.
In U.S. Pat. No. 4,053,227 which issued on Oct. 11, 1977 to B. Bodlaj, a method and apparatus is disclosed for measuring the height of a blade of a turban rotor as the rotor is being rotated without any contact taking place. A laser beam is deflected along a given path at a known frequency of deflection, the laser beam impinges on the end of the moving blade wherein a portion of the laser beam is reflected from the end of the blade. A portion of the reflected laser beam is sensed to provide a signal, the signal and a reference signal from the laser beam is provided to an electronic analysis system to determine the height of each individual blade.